Industrial processes often require measuring the level of liquid or other material in a tank. Many technologies are used for level measurement. With contact level measurement some part of the system, such as a probe, must contact the material being measured. With non-contact level measurement the level is measured without contacting the material to be measured. One example is non-contact ultrasound, which uses high-frequency sonic waves to detect level. Another example is use of high-frequency or microwave RF energy. Microwave measurement for level generally uses either pulsed or frequency modulated continuous wave (FMCW) signals to make product level measurements. This method is often referred to as through air radar. Through air radar has the advantage that it is non-contact and relatively insensitive to measurement errors from varying process pressure and temperature. Known radar process control instruments operate at frequency bands of approximately 6 Ghz or 24 Ghz.
A through air radar measurement instrument must convert a high frequency electrical signal to an electromagnetic wave. A conventional device for accomplishing this is an exciter element or loop launcher housed in a waveguide. A loop launcher is a wire which couples energy from a coaxial feeder cable into the waveguide. Problems can occur in creating impedance matching between the coaxial cable and a required propagating mode, such as the TE11 mode. An antenna, such as a dielectric rod or horn, is operatively associated with the waveguide. An ultra-high frequency radiation beam is propagated downward from the antenna, and reflected off the surface of the material being measured to the antenna where the signal is received. The product level is calculated from the total time of propagation of the beam.
Typically, the antenna, waveguide and like components are provided as a unit and installed in a process vessel. If it is necessary to service the electronics, launcher or waveguide, then the device must be removed which disturbs the process seal. This can be problematic, particularly in hazardous process environments.
A difficulty can be encountered when a metal object is located in or around the radiated electromagnetic field. A reflection from a metal object can cause a false target situation, in which the system evaluates the product to be at a level indicated by the reflected signal from the object and not from the actual product. Typical false target objects in tanks are mixers, nozzles, ladders and tank walls. The radiated electromagnetic field distribution can be asymmetric about its centerline, and the pattern may vary along the direction of propagation. This phenomenon can be used as an advantage against false target detection. A signal level from a false target can vary as the antenna is rotated about its vertical axis. More specifically, the orientation of the loop launcher in the waveguide determines the sensitivity of the system to a false target. Optimization of the launcher position, as by rotating the device about the vertical axis, can minimize the effect of false targets.
The present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner.